1. Technical Field
The present disclosure relates to a method of matching dynamic elements and multi-bit data converters and, more particularly, to a method of matching dynamic elements and multi-bit data converters capable of solving an in-band tone problem caused by a repeated selection of a unit element during a conversion of multi-bit data.
2. Discussion of the Related Art
Techniques for implementing a mixed-mode signal processing (MMSP) integrated circuit (IC) on a single chip are being developed. MMSP includes processing of both analog and digital signals. Also, data converters, for example, an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC), are required to have a high resolution.
Different types of data converters include, for example, a Nyquist-based data converter that applies a sampling technique using a Nyquist rate, and an oversampling data converter that applies an oversampling technique.
The Nyquist-based data converter may operate at high speed because the converter samples an input signal with the Nyquist rate. A high resolution is difficult to achieve in the Nyquist-based data converter, however, because the Nyquist-based data converter needs an analog passive element having a high precision and is vulnerable to noise or signal interference.
Therefore, the oversampling data converter is generally used for data conversion of a high resolution. The oversampling data converter needs high-speed operation and complicated digital signal processing since the oversampling data converter uses a high sampling frequency. The digital signal processing may be implemented in the oversampling data converter, however, by using an analog element that needs comparatively less accuracy.
Among the different types of oversampling data converters, a delta-sigma data converter that performs noise shaping in a signal band is widely used for signal processing in an audio signal band that requires a high resolution due to a narrow frequency range thereof.
FIG. 1 is a block diagram illustrating a conventional delta-sigma data converter having a single-bit modulator.
Referring to FIG. 1, the delta-sigma data converter 10 includes a single-bit modulator 11, a 1-bit digital-to-analog converter (DAC) 13 and a low-pass filter 15.
When digital data is inputted to the single-bit modulator 11, the single-bit modulator 11 converts the inputted digital data into a 1-bit sigma-delta signal, and then outputs the converted signal. The outputted 1-bit sigma-delta signal is converted into a continuous time signal through the 1-bit DAC 13. The low-pass filter 15 performs filtering of the continuous time signal, which is outputted from the 1-bit DAC 13 and passes a necessary band to output an analog signal.
Because the delta-sigma data converter 10 includes the single-bit modulator 11 that performs a quantization using only two steps, the linearity of the conversion process is guaranteed. A high-degree modulator for a high resolution, however, may cause a problem of low stability.
Therefore, a multi-bit modulator is typically used in the delta-sigma data converter so as to solve the problem of low stability. The multi-bit modulator has enhanced stability compared with the single-bit modulator. Thus, the multi-bit modulator may have a high signal-to-noise ratio (SNR) with a relatively low oversampling ratio (OSR).
The delta-sigma data converter using the multi-bit modulator requires a multi-bit DAC. In this case, the delta-sigma data converter may have a problem related to a mismatch between unit elements in the DAC.
Each bit of a digital input code is converted to an analog signal by switching a corresponding analog unit element, such as a capacitor, and then the converted analog signals are summed and outputted. Fluctuations between the respective unit elements, that is, a mismatch error, may cause a nonlinearity of the digital-to-analog conversion.
A linear error in an analog to digital converter (ADC) may be pushed out of the signal band by noise shaping. The linear error in the DAC, however, degrades efficiency of the whole system, because the linear error is located at the same position as the signal from the aspect of the transmission function of the whole system.
Therefore, research on dynamic element matching (DEM) is being conducted in an effort to solve the problem of mismatch errors between unit elements in the converter. For example, use of the DEM technique is disclosed in U.S. Pat. No. 5,990,819, entitled “D/A converter and delta-sigma D/A converter.”
According to the above DEM technique, the mismatch error between elements may be converted into white noise in the frequency domain by randomly selecting unit elements according to each DAC operation.
In addition, by using a recurring algorithm, such as a data weight averaging (DWA) technique that selects at least one unit element based on an inputted digital signal so as to average the mismatch errors between unit elements, noise shaping of the noise caused by the mismatch error in the signal band may be accomplished.
Fundamental techniques of the DWA technique are disclosed, for example, in a publication by Rex T. Baird and Terry S. Fiez, entitled “Linearity enhancement of multibit ΔΣ A/D and D/A converters using data weighted averaging, IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, Vol. 42, No. 12, December 1995.”
The conventional DWA algorithm, however, selects unit elements in sequential order or by simply changing the order of selection based on an inputted digital signal and may cause an in-band tone problem when a specific signal is repeatedly inputted.
In the recurring algorithm, such as the DWA, a mismatch error of the DAC is converted into broadband noise. Periodic signal elements, that is, tones, however, are generated at a specific frequency.
The generation of the tones is not desirable, because the tones have a tendency to reduce the dynamic range of the system. The generated tones demodulate noise that is outside of a desired signal band and interrupt signals within the desired signal band. Although the tones cause little noise, the noise may still be audible in the case of an audio converter. More particularly, the smaller the size of the inputted digital data, the more serious the problem of the in-band tone becomes.